In semiconductor lasers, the unpumped cladding region outside the optical waveguide is nominally lossy. Since the evanescent tail of higher-order waveguide modes extends further into the waveguide cladding, the modal loss increases for high spatial frequency perturbations, creating a weak spatial filter. Increasing the effective laser cavity length in a broad-area laser enhances the effect of this spatial filter, resulting in improved beam quality. Simulations predict that by optimizing the lateral index step and output coupler reflectivity, a factor of two increases the beam brightness with only a 10% penalty in electrical-to-optical efficiency.